Medical electrical lead and reinforced silicone elastomer compositions used

ABSTRACT

A medical electrical lead and a reinforced silicone elastomer used therein. The silicone elastomer used therein is preferably made from a novel silica reinforced polysiloxane material, which after vulcanization by cross-linking exhibits improved mechanical properties. The medical electrical lead features a an electrode at a distal end thereof, a connector at a proximal end thereof and an elongated electrical conductor extending between the electrode and the connector, the conductor in electrical contact with the electrode at a distal end and in electrical contact with the connector at a proximal end, the conductor comprised of a plurality of wires or wire bundles wound in a multifilar coil configuration.

This application is a division of application Ser. No. 08/697,991, filedSep. 4, 1996, now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention is directed to reinforced polysiloxane elastomercompositions of improved mechanical properties. More specifically thepresent invention is directed to reinforced polysiloxane elastomercompositions which have improved creep resistance, improved compressionset and improved crush resistance and are eminently suitable for use asinsulators for leads of implantable medical devices, particularlycardiac pacemakers.

2. Brief Description of the Prior Art

Polysiloxane elastomer compositions have been known in the art for along time. Many polysiloxane elastomer compositions of the prior artcontain a silica reinforcer that has been treated to make it compatiblewith the generally hydrophobic polysiloxane matrix of the elastomer.Prior art pertaining to polysiloxane materials can be found in U.S. Pat.Nos. 3,341,490, 3,284,406, 3,457,214, 3,996,187, 3,996,189, 4,615,702,5,236,970 and in European Patent application No. 0110537 filed on Oct.18, 1983.

Silicone elastomer rubber tubing that contains silica reinforcingmaterial has been extensively used as insulator on electrical leads,particularly on leads of implantable medical devices, perhaps mostimportantly from the standpoint of the present invention, on leads ofcardiac pacemakers. An early version of the reinforced elastomer used asinsulators on implantable devices, primarily pacemakers, was known inthe trade as Dow Corning MDX4-4516. A later version was known as DowCorning HP (high performance) material. The early version (MDX4-4516)consisted of silica reinforced polydimethylsiloxane which includedapproximately 0.142 mol per cent methylvinylsiloxane (CH₃)(CH₂═CH)Si--O! units. The end blocking (terminal) units of this earlyversion of the polymer were dimethylvinylsiloxane units (CH₃)₂ (CH₂═CH)Si--O!. Based on the presence of approximately 0.142 mol percent ofmethylvinylsiloxane units in this prior art polymer, it can becalculated that approximately 750 dimethylsiloxane (CH₃)₂ SiO! units aredisposed on the average between each "pendant" vinyl group in thepolysiloxane chain. The vinyl groups in the polymer participate in across-linking reaction which occurs in the final curing step when thepolymer is formed into a desired shape, such as a single or multi-lumentubular object. The curing or cross-linking step was initiated byaddition of a peroxide catalyst or catalysts. The filler material ofthis earlier version was silica that had been treated withdimethylsiloxane oligomer.

The later, high performance version of the prior art silicone elastomer(Dow Corning HP) used as leads in implantable medical devices, primarilypacemakers, consisted of a blend of a first (major) and a second (minor)polysiloxane composition. The first and major composition comprisedapproximately 80-90 per cent (by weight) of the blend, and thiscomposition contained no pendant vinyl or other pendant olefinic groups.The second and minor composition comprised the balance of the blend(before reinforcing treated silica was added), and had approximately 2mol percent methylvinylsiloxane (CH₃)(CH₂ ═CH)Si--O! units and was alsoterminated with dimethylvinylsiloxane (CH₃)₂ (CH₂ ═CH)Si--O! units. Thereinforcing material was silica that had been treated with a reagentthat introduced trimethylsilyl groups into the material, therebyreplacing OH functions with OSi(CH₃)₃ functions and rendering thetreated silica compatible with the "hydrophobic" silicone polymer. Thecuring or cross-linking step was initiated by addition of a platinumcatalyst or catalysts.

As it will be readily appreciated by those skilled in the art, certainmechanical properties, such as tear strength, abrasion resistance,resistance to shredding, compression set, crush and creep resistance areof great importance in the materials for electrical leads in any devicethat is implanted into the human body, and particularly so forinsulators of cardiac pacemakers. It should also be readily appreciatedby those skilled in the art that improved mechanical properties, andparticularly improved compression set, creep and crush resistance allowthe manufacture of insulated electrical leads of smaller dimensions andtherefore facilitates "downsizing" of the implantable device. Whereasthe later version (Dow Corning HP) of the above-summarized prior artsilicone elastomers had certain improved mechanical properties (forexample improved tear strength) relative to the earlier MDX4-4516version, it was then surprising to the artisans in the field that thisotherwise improved material had less crush resistance than the earlierMDX4-4516 material. Therefore, up to the present invention the prior artstruggled with the problem that neither of the two types of siliconeelastomeric materials available for forming insulators for cardiacpacemaker and similar implantable leads had optimal characteristics. Asnoted above, the earlier version could have used improvement in severalmechanical properties, and the later version had improved mechanicalproperties in virtually all aspects, but had less crush resistance thanthe earlier version.

In light of the foregoing, up to the present invention the need stillexisted in the prior art for a polysiloxane elastomer material which issuitable for use as insulator for leads of implantable electricaldevices, particularly cardiac pacemakers, and which has improved overallmechanical properties, including improved compression set, and improvedcrush and creep resistance. The present invention provides such amaterial.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a silicone elastomermaterial which is suitable for forming insulation for electrical leadsof implantable medical devices, and which has improved overallmechanical properties.

It is another object of the present invention to provide insulatingsingle-lumen or multi-lumen tubular material for electrical leads ofimplantable medical devices, especially cardiac pacemakers, whichmaterial has improved overall mechanical properties, including improvedcompression set, and improved crush resistance.

The foregoing and other objects of the present invention are attained byan elastomer composition that, before a cross-linking step of finalcuring, essentially consists of the following materials:

approximately 23 to 45 percent by weight of silica that had beensilyliated by treatment;

the balance of the composition being a polysiloxane copolymer composedof divalent --R₁ R₂ SiO--, divalent --R₃ R₄ SiO-- and monovalent(terminal, or end-blocking) R₅ R₆ R₇ SiO-- units,

where R₁ and R₂ independently are lower alkyl of 1 to 6 carbons, phenylor trifluoropropyl,

R₃ is vinyl, allyl, or other olefinic group having up to 4 carbons, R₄is lower alkyl of 1 to 6 carbons, phenyl or trifluoropropyl, and

R₅, R₆, and R₇ independently are lower alkyl of 1 to 6 carbons, phenyl,vinyl, allyl, or other olefinic group having up to 4 carbons.

The above-described polysiloxane copolymer has a degree ofpolymerization (D. P.) approximately in the range of 3500 to 6500, andthe olefin containing --R₃ R₄ SiO-- groups are randomly distributed inthe copolymer and are present approximately in the 0.05 to 0.3 molpercent range. The balance of the polysiloxane copolymer composition ismade up of the above noted divalenty --R₁ R₂ SiO-- siloxane units, andof the end-blocking R₅ R₆ R₇ SiO-- units. However, to the extent R₁, orR₂, or both may represent phenyl groups, the proportion of thephenyl-containing divalent siloxane units does not exceed 15 mol percent. To the extent R₁ or R₂ represents trifluoropropyl groups, theproportion of the trifluoropropyl-containing divalent siloxane unitsdoes not exceed approximately 40 mol per cent in the polysiloxanecopolymer of the invention. In such situations in the balance of the--R₁ R₂ SiO-- siloxane units the substituents are neither phenyl, nortrifluoropropyl substituted, respectively.

Those skilled in the art will readily understand that the proportion ofthe end-blocking R₅ R₆ R₇ SiO-- units, as expressed in mol percentage,is determined by the degree of polymerization: when the degree ofpolymerization is 5000, the end blocking groups are present as 2/5000mol per cent.

The above-described composition undergoes a cross-linking or "finalcuring" step after a platinum catalyst, an organohydrogen polysiloxanecross-linker and a suitable inhibitor are added. The composition thatexhibits the desired improved mechanical/physical properties is a resultof the cross-linking or final curing reaction. In the cured compositioncovalently linked intermolecular and intramolecular ethylenic (--CH₂--CH₂ --) bridges are formed from some of the olefin groups of thepolysiloxane copolymer. The improved mechanical/physical propertiesinclude high tear, abrasion creep and crush resistance, and improvedcompression set. The cured silicone material is resistant to crackinitiation under compressive loading. The objects consisting of thecomposition of the present invention, such as the single or multi-lumentubes that are used as insulators for leads of cardiac pacemakers (or ofother implantable medical devices) can be formed by means known in theart, such as extrusion or molding. The final curing step occurs when thecomposition has already been formed into substantially the desiredshape.

The features of the present invention can be best understood togetherwith further objects and advantages by reference to the followingdescription, taken in connection with the accompanying drawing.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a plan view of a medical electrical lead system suitable forendocardial stimulation by an implantable heart pacemaker.

FIG. 1a is a cross-sectional view of a lead assembly portion of the leadsystem of FIG. 1.

FIG. 2 is a schematic showing of an extrusion line for manufacturingthin walled medical tubing of silicone elastomer.

The drawings are not necessarily to scale.

DETAILED DESCRIPTION OF THE INVENTION

Reinforced elastomeric compositions are provided in accordance with thepresent invention which after curing by cross-linking are suitable foruse as insulators for electrical leads, and particularly as insulatorsfor medical electrical leads, especially cardiac pacemaker leads.

In a preferred embodiment of the present invention, a medical electricallead which features a reinforced elastomeric composition comprises anelectrode at a distal end thereof, a connector at a proximal end thereofand an elongated electrical conductor extending between the electrodeand the connector, the conductor in electrical contact with theelectrode at a distal end and in electrical contact with the connectorat a proximal end, the conductor comprised of a plurality of wires orwire bundles wound in a multifilar coil configuration.

Referring now to the drawings, FIG. 1 shows a lead system 10 whichincludes a lead assembly 15, an anchoring sleeve 20, a connector 25, astylet guide 30, and a stiffening stylet 35. As is well known in theart, once implanted stylet guide 30, and a stiffening stylet 35 areremoved from lead.

Referring now to FIG. 1a, the lead assembly 15 is shown in greaterdetail with an electrode structure 40 at a distal end of the leadassembly 15, a tine 45 to secure the lead assembly 15 to theendocardium, a lead conductor 50 in a multifilar coil configurationwhich allows the stiffening stylet 35 to be inserted into the leadassembly 15 in the internal lumen 52 of the lead conductor 50. The leadconductor 50 is shown attached at its distal end 55 to the electrodestructure 40. The lead conductor 50 is also similarly attached at aproximal end (not shown) to the connector 25. In the preferredembodiment conductor 50 is a multifilar coil. Insulation elements 57a,57b and 57c insulate portions of the electrode structure 40 and the leadconductor 50. Such insulation elements 57a, 57b, and 57c are preferablymade from any of the reinforced polysiloxane elastomer compositionsalready described above. The insulator 57c is typically a hollowpolymeric tube extending between the proximal and distal ends of thelead assembly 15 and insulating the lead conductor 50 from surroundingbody tissues. Housed within insulator at the distal end is a monolithiccontrolled device 99 to elute an anti-inflammatory agent, as is wellknown in the art. While a unipolar lead is shown, and described above,the present invention can also be applied to bipolar leads in the samemanner. As used in implantable pacing leads, the individual wires of thelead conductor 50 would be typically about 0.004 to 0.010 in diameterand would be wound into extremely small coils; typically having adiameter of less than 23 turn.

Turning now to the elastomeric composition of the present invention,before final curing or cross-linking, includes a polysiloxane copolymerthat has the following composition and characteristics. The polysiloxanecopolymer has a degree of polymerization of approximately 3500 to 6500,with a D. P. of approximately 5000 being preferred. Those skilled in theart will readily understand that the actual extent or degree ofpolymerization of a polysiloxane product is very difficult, if notimpossible, to measure. Therefore, the "degree of polymerization"normally used in the art to characterize a polysiloxane product isusually based on theoretical considerations and measurements whichmeasure a physical characteristic (such as plasticity) that is relatedto D. P. Therefore, in accordance with usual practice in the art thedesignated degree of polymerization of a polysiloxane material is thatwhich would be normally expected based on the materials and conditionsused in the polymerization reaction and on measurements of certaincharacteristics, such as plasticity.

The polysiloxane copolymer includes divalent --R₁ R₂ SiO-- and divalent--R₃ R₄ SiO-- units where the R₁ and R₂ groups independently are loweralkyl of 1 to 6 carbons, phenyl or trifluoropropyl. Preferably, the R₁and R₂ groups both are methyl. Therefore in the preferred embodiment ofthe composition of the invention the --R₁ R₂ SiO-- unit representsdimethylsiloxane (CH₃)₂ SiO!.

R₃ is vinyl, allyl, or other olefinic group having up to 4 carbons, andR₄ is lower alkyl of 1 to 6 carbons, phenyl or trifluoropropyl.Preferably the R₃ group is vinyl and the R₄ group is methyl. Thereforein the preferred embodiment of the composition of the invention the --R₃R₄ SiO-- groups represents methylvinylsiloxane CH₃ (CH₂ ═CH)SiO!. Thepresence of the vinyl (or other olefinic group designated R₃) isimportant for the present invention because these "pendant" vinyl (allother olefinic) groups enable the cross-linking reaction which occursduring final curing, and which is described later. The --R₃ R₄ SiO--groups, or more preferably the methylvinylsiloxane groups, are presentin the range of approximately 0.05 to 0.3 mol percent, preferablyapproximately 0.142 mol per cent. Except for the end-blocking groupsdescribed below, the balance of the polysiloxane copolymer consists ofthe --R₁ R₂ SiO--, preferably dimethylsiloxane, groups which thus formthe backbone or bulk of the polysiloxane chain.

The nature of the end-blocking or terminal groups R₅ R₆ R₇ SiO-- is lesscritical from the standpoint of the present invention than in the priorart. The R₅, R₆, and R₇ groups independently are lower alkyl of 1 to 6carbons, phenyl, vinyl, allyl, or other olefinic group having up to 4carbons. It is noteworthy, that unlike in the prior art the end-blockinggroup does not need to necessarily include a vinyl or other olefinicgroup, although in the preferred embodiment the end blocking group isdimethylvinylsiloxane (CH₃)₂ (CH═CH₂)SiO!. The proportion (expressed inmol percent) of the end-blocking groups relative to the entirepolysiloxane copolymer is determined by the degree of polymerization ofthe substance. With a preferred D. P. of 5000, the mol percent of theend blocking groups is 2/5000. Because the range of D. P. of thepolysiloxane copolymer is approximately 3500 to 6500 in accordance withthe invention, the mol percentage of the end blocking groups is in therange of approximately 2/6500 to 2/3500 (approximately 0.00031 to 0.0006mol percent).

It follows from the foregoing data that the mol percentage of the --R₁R₂ SiO--, preferably dimethylsiloxane, groups in accordance with theinvention groups (calculated to two decimal points) is in the range ofapproximately 99.7 to 99.95 mol percent. However when the R₁ or R₂symbols, or both, represent phenyl groups then the proportion of thephenyl-containing divalent siloxane units does not exceed 15 mol percent, the balance of the --R₁ R₂ SiO-- groups do not include a phenylgroup. A similar restriction of approximately 40 mol per cent applieswhen the R₁ R₂ SiO-- group includes trifluoropropyl.

The above described composition of the polysiloxane copolymer is noveland surprising especially in terms of the mechanical propertiesachieved. This is because, as is noted in the description of the priorart, the prior art MDX4-4516 copolymer having "pendant" vinyl groupslacked somewhat in mechanical properties such as tear resistance. Theother prior art HP product which had pendant vinyl groups only in aminor component of two blended copolymers, had improved tear resistancebut decreased crush resistance. Therefore, it is surprising that thepresent composition that includes pendant vinyl groups in the single andtherefore major macromolecular component of the elastomer, has increasedcompression set, and improved creep, crush and abrasion resistance.

Another major component of the elastomeric composition of the presentinvention is silyliated silica that acts as a reinforcer in thecomposition. Blending silyliated "fumed silica" into a polysiloxanecopolymer as a "reinforcer" and to improve its mechanical properties,per se is not new in the art. In accordance with the present inventionthe fume silica used as a reinforcer is treated with a reagent thatintroduces trialkylsilyl groups (alkyl has 1 to 6 carbons, preferably 1to 2) to the surface of the silica so that a plurality of OH functionson the surface become O(trialkylsilyl) functions. Preferably,trimethylsilyl groups are introduced into the silica used in thecomposition of the present invention. The silica used in the presentinvention has a relatively small surface area, as this term isunderstood in the art, nevertheless the surface area should be at leastapproximately 200 meter² /gram of the yet not silyliated silica. Thesilyliation, preferably introduction of trimethylsilyl groups, can beaccomplished before the silica is admixed or blended with thepolysiloxane copolymer that has been described above. Alternatively, thesilyliation of the silica may be performed as part of the blendingprocess of the copolymer with the silica. In the preparation of thepreferred embodiment of the elastomer of the present invention thelatter procedure is preferred. In any event, the degree of silyliationis normally measured in the art by expressing the carbon content of thesilyliated silica. (As is known, the carbon content can be readilydetermined analytically, for example by combustion analysis.) Inaccordance with the present invention the carbon content of thetrimethyl silyl groups containing silica should be in the range ofapproximately 4 to 8 per cent by weight of the silyliated silica, andpreferably approximately 7.3 per cent by weight. The amount of treated(trimethylsilyliated) silica used in the composition is in the range ofapproximately 23 to 45 per cent by weight of the composition, withapproximately 39 per cent by weight being preferred.

The elastomeric composition which is the result of blending treatedsilica and the above-described polysiloxane copolymer together is notcross linked (not yet cured or vulcanized) and therefore does not yethave the desired physical/mechanical properties. Nevertheless this blendor composition (sometimes referred to as the "base") itself isconsidered innovative and useful because it serves as a precursor to thecured elastomer that has the advantageous mechanical properties. Thusthe uncured composition or base has the inherent characteristics ofproviding, after suitable vulcanization by cross-linking, a materialthat can be shaped to form tubular insulators for pacemaker leads,having excellent mechanical properties.

Normally the final curing and vulcanization step is performed only afterthe blend or composition is shaped into the desired object (for exampleby extrusion or molding) and this step of fabrication and vulcanizationis normally performed at locations different than where the manufactureof the blend occurs. For these reasons, in accordance with standardpractice in the art, the blend composition or base is divided into twosubstantially equal weight and volume aliquots, designated "Part A" and"Part B". Suitable catalyst to catalyze the cross-linking reaction isadded to one of the aliquots (Part A), and a cross-linking agent and across-linking inhibitor is added to Part B. The two parts are kept andtransported separately and are intimately mixed together just prior tofabrication of the desired object. The amounts of catalyst,cross-linking agent and inhibitor are "fine tuned" in the preferredembodiment of the present invention to provide appropriate time for thefabrication of insulators for cardiac pacemaker leads and similarobjects.

As is known in the art, the cross-linking (curing or vulcanization step)is the result of a platinum catalyzed reaction between a silicon bonded"pendant" and/or terminal vinyl (or other olefinic group) and a siliconbonded hydrogen group. The vinyl (or other olefinic) group is present inthe polysiloxane composition even before it is divided into Parts A andParts B. The silicon bonded hydrogen group is present in the blend ofthe two parts because it is added to Part B in the form of across-linking agent. In the actual cross-linking reaction ethylenic(--CH₂ --CH₂ --) bridges are formed by saturating vinyl groups andlinking one polymer molecule to a silicon atom of a cross-linkingmolecule, which is in turn linked by yet another ethylenic bridge (bysaturating another vinyl group) to another polymer molecule. In essencethe chemical reaction, which is per se known in the art, involvessaturation of a vinyl (or other unsaturated) group of an R₃ R₄ SiO unitand/or of a terminal R₅ R₆ R₇ SiO unit with the hydrogen derived from anat least difunctional organohydrogen polysiloxane, and formation ofcarbon to silicon bonds and thereby bridges between the severalpolysiloxane molecules.

More specifically, the platinum catalyst can be selected, within theskill of the art, primarily from organo platinum compounds, for examplein accordance with U.S. Pat. Nos. 2,823,218 and 3,159,601 which areincorporated herein by reference. The platinum catalyst is added to PartA in amounts of approximately 6 to 30 parts per million as platinum,with the result that in the blend of Part A and Part B which is to bevulcanized or cured the platinum is present in the proportion ofapproximately 3 to 15 part per million per weight.

Although, as noted above the platinum catalyst can be selected withinthe skill of the art, in the preferred embodiment a platinum catalyst isused which is the result of complexing tetramethyldivinyldisiloxane withhexachloroplatinic acid pentahydrate, and this complex is added to PartA in the proportion of 10 to 24 part per million (ppm) per weight asplatinum, preferably 20 ppm. A number of cross-linking agents aresuitable for the practice of the present invention and can be selectedby those familiar with the art. U.S. Pat. No. 3,436,366 describes anumber of cross-linking agents and its specification is expresslyincorporated herein by reference. Thus, the liquid organohydrogenpolysiloxane cross-linkers shown in Column 2 of the above noted U.S.Pat. No. 3,436,366 and having the formula (R)_(a) (H)_(b)SiO_(2-a/2-b/2) where R is simple lower alkyl and "a" ranges from 1.00to 3, and "b" ranges from 0.1 to 1.0 are particularly satisfactory foruse in the present invention. Especially suitable is the cross-linker ofColumn 4, lines 3-14 of the 366 patent which has the formula R₂HSiO_(1/2) where the R groups are primarily or predominantly methyl. Thecross-linking agent is added to the second aliquot (Part B) of thecomposition in the proportion of approximately 2 to 6 parts per hundredper weight of Part B. Consequently in the blend of Part A and Part Bwhich is to be vulcanized or cured the cross-linking agent is present inthe ratio of approximately 1 to 3 parts per hundred per weight.

It is important in accordance with the present invention that, aftermixing the aliquots (Parts A and B) the cross-linking reaction notproceed too rapidly at room temperature, allowing at least a few hoursand even up to 6-8 hours for work time with the mixed aliquots. For thisreason one or more suitable inhibitors of the cross-linking reaction arealso added to the mixture. Preferably the inhibitor is added to the PartB aliquot. Suitable inhibitors may be readily selected within the skillof the art. One example of such an inhibitor is1,2,3,4-tetramethyl-1,2,3,4-tetravinyl cyclotetrasiloxane, another is1-ethynyl-1-cyclohexanol. The ethynylcyclohexanol inhibitor is added toPart B in the range of approximately 0.01 to 0.2 parts per hundred partsof Part B, by weight. Therefore it is present in the mixed aliquots tobe cured or vulcanized in the range of 0.005 to 0.1 part per hundred,per weight. This inhibitor, being volatile, can be driven off by heat ina final curing step, thus allowing the cross-linking reaction to occurrapidly. Other less-volatile or non-volatile inhibitors may be usedinstead, the amount of these would be adjusted such that thecross-linking reaction should nevertheless occur in the desired timeframe after Part A and Part B are mixed.

Addition of catalyst, cross-linker and inhibitor in the above-notedranges usually serves to provide approximately 6-8 hours of work time atroom temperature. This means that the material does not curesignificantly at room temperature within 6-8 hours. Before curing andcross-linking the two aliquots are intimately mixed preferably in equalamounts.

The process of manufacturing the cured reinforced polysiloxane objects,which can serve as insulators for leads of cardiac pacemakers or otherdevices, starting with the preparation of the preferred embodiment ofthe polysiloxane copolymer, thus proceeds as follows.

In a suitable reactor octamethylcyclotetrasiloxane (precursor to the--R₁ R₂ SiO-- groups of the copolymer) andtetravinyltetramethylcyclotetrasiloxane (precursor to the --R₃ R₄ SiO--groups of the copolymer) are mixed and heated until a temperature of150° C. is reached, and the mixture is stirred at that temperature forapproximately 1 hour. Then the end blocker (precursor to the R₅ R₆ R₇SiO-- units of the copolymer) is added together with a catalyst. The endblocking reagent or material can be tetramethyldivinyidisiloxane, andthe catalyst can be acid or base catalyst normally utilized in the artfor siloxane polymerization reactions, as is described for example inU.S. Pat. No. 3,779,987 incorporated herein by reference. Preferably,however the end blocking reagent is a product which results from thereaction (equilibration) of tetramethyldivinyldisiloxane withoctamethylcylcotetrasiloxane units. The catalyst is preferably potassiumsiloxanolate which per se is known in the art. After the end blockingreagent and the catalyst are added, heating is continued only for ashort time, whereafter the catalyst is neutralized. In case of potassiumsiloxanolate the catalyst is neutralized by introducing carbon dioxideinto the mixture. Although as it was noted above the degree ofpolymerization per se is hard or impossible to measure, plasticity isrelated to D. P. and can be measured in the routine test known as ASTM926 "Rubber Property-Plastics and Recovery (parallel plate method). Thepolymerization reaction can thus be monitored by this test. At the endof the polymerization (corresponding to a desired theoretical D. P. ofapproximately 5000) the plasticity should be approximately 55, with arange of 50 to 60 being acceptable. The just described polymerizationreaction provides the polysiloxane copolymer component of the elastomerof the present invention.

The polymer is subjected to vacuum to remove volatile materials, and isthereafter mixed with the requisite amount of silica. As it was notedabove, in the process preferred for making the elastomer of the presentinvention, the silica is silyliated substantially in the same processwhere it is blended with the polysiloxane copolymer. Thus, in thepreferred process silica having a surface area of at least 200 m² /gramis slowly blended under an inert gas blanket into the polysiloxanecopolymer obtained above, together with water and the silyliating agenthexamethyidisilazine (CH₃)₃ Si--N(H)--Si(CH₃)₃ !. The mixture is heatedfirst under the blanket of inert gas, and thereafter in vacuum to removevolatile materials. The resulting silica reinforced polysiloxanecopolymer composition is sometimes referred to as the "base". It is thiscomposition or base which is divided into the aliquots called Part A andPart B. The catalyst, cross-linking agent and inhibitor are added toPart A and Part B, respectively by intimate mixing, for example on a tworoll mill, followed by passing the mixture through fine stainless steelmesh screens. Typical mesh size is generally in the range of 200 to 500,preferably approximately 400 mesh.

Fabrication of objects from the two aliquots is accomplished byintimately mixing equal or substantially equal volumes and weights ofthe aliquots and then shaping the mixture into the desired form, beforethe cross-linking or curing reaction is substantially completed. Thecross-linking reaction can be accelerated by placing the shaped objectinto a hot air vulcanizing chamber where a volatile inhibitor is drivenoff, thereby allowing the cross-linking reaction to occur rapidly.

Generally speaking, Parts A and B are mixed on a two roll mill whichprovides the blended base where the curing is inhibited due to presenceof the inhibitor. The blended base is then extruded through standardextrusion techniques which are per se known in the art, and areillustrated in the FIG. 2. Thus the mixed aliquots (blended Parts A andB) are placed into the extruder device 100. A typical tubing line alsoincludes a die 120, a curing or vulcanizing oven 140, and a pullerwinder device 160 on which the extruded tubing is taken up or coiled.The die 120 preferably includes an internal mandrel (not shown) as iswell known in the art. To obtain precise dimensions such as may benecessary with small thin-wall medical tubing, a gear pump (not shown)may be used at the extruder discharge for stable throughput. Gear pumpsare calibrated to forward precise metered amounts of polymer. Their usein this manner is well known. After the tubing is formed by extrusion itis placed into a hot air vulcanizing chamber where the volatileinhibitor (ethynylcyclohexanol) is driven off, thereby allowing thecross-linking reaction to proceed to yield the final product. Thepreferred method for mixing Parts A and B, before shaping the mixtureinto insulators for leads of implantable medical devices by extrusion isdescribed in more detail below as a specific example.

A creep evaluation test was performed on the preferred embodiment of thecured elastomer composition of the present invention which has beenshaped into the form of tubing, in accordance with ASTM D 2990 StandardTest Methods for Tensile, Compression, and Flexural Creep and CreepRupture of Plastics. For comparison the same tests were also performedon the prior art materials discussed in the introductory section of thisapplication, namely on MDX4-4516 and on the HP material. The percentageof initial elongation upon loading was the highest for the prior artMDX4-4516 material (approximately 225%) less for the HP material(approximately 150%), and still less for the preferred embodiment(approximately 110%). Percent elongation upon loading changed in timethe most (the slope of the elongation versus time curve was the largest)with the HP material, less with the MDX4-4516 material, and the leastwith the preferred embodiment of the present invention.

Crush resistance testing was performed by dynamically compressing thesilicone tubing under a defined consistent per cent compression relativeto the cross-sectional width of the tubing. The cycles of thecompression were measured and recorded until the tubing split. Alltubings in these tests had the same inner and outer diameters and wallthickness. In these tests the prior art HP material split afterapproximately 500 cycles (least crush resistant), and the prior artMDX4-4516 material split after 1500 cycles. The preferred embodiment ofthe invention split only after 2800 cycles, which represent a 460%increase over the HP material, and still a significant 86.6% increaseover the MDX4-4516 material.

SPECIFIC EXAMPLE

Preparation of Base Polymer

In a 150 gallon suitable mixer, mix octamethylcyclotetrasiloxane (300.0kg) and tetramethyltetravinylcyclotetrasiloxane (0.50 kg) and heat to150° C. with agitation under a nitrogen blanket. Hold at 150° C. for onehour. After one hour add pre-equilibrated vinyidimethyl-terminatedsiloxane oligomer (450.0 grams) as end-blocker and 0.001 percent (byweight) of potassium siloxanolate catalyst (about 300 grams). Continueheating and stirring until polymerization is completed. (about 3 min).After completion of the polymerization the catalyst is then neutralizedor destroyed by bubbling CO₂ through the polymer while continuingmixing. At this time the polymer is devolatilized by pulling a fullvacuum on the mixer. Continue the vacuum for one hour while continuingto bubble CO₂ through the polymer. After one hour, stop the vacuum, stopCO₂ flow, vent the mixer with N₂, and allow to cool.

Formulation of Base Including Silica Reinforcer

In a 50 gallon, Sigma blade mixer, mix the above polymer (90.0 kg) withhexamethyidisilizane (8.06 kg) and water (2.30 kg). Blanket the mixerwith N₂. Add adequate fumed silica to fill the mixer. Mix until thepolymer and silica has massed. Continue in this manner until the totalamount of silica has been added (57.6 kg). After the final silicaaddition has massed, bring the base to 80° C. and hold at thistemperature for 30 minutes. After 30 minutes, stop N₂ flow, turn onvacuum to mixer, and begin heating to 180° C. Continue mixing for threehours after temperature reaches 180° C. while maintaining full vacuum.After three hours, vent mixer with N₂ and allow to cool.

Preparation of Parts A and B

The base is now divided into two equal parts. One of the two parts isnow softened on a two roll mill. After softening, a platinum complexcatalyst is added via two roll milling such that the resulting Part Acontains 10-24 ppm platinum, typically 20 ppm. Using a screw typeextruder, the Part A is passed through fine stainless steel meshscreens. Mesh size is generally 200-500, typically 400 mesh. The secondhalf of the base is similarly softened. After softening, bothcross-linker and inhibitor are added via two roll milling. The siloxanecross-linker, a copolymer consisting of both dimethyl and methylhydrogenmonomers, is added at 2-4 parts per hundred (pph) by weight, andtypically 3.0 pph. Similarly the inhibitor, generally an acetylinicalcohol such as 1-ethynyl-1-cyclohexanol, is added at amounts up to 0.12pph, typically at 0.08 pph. The resulting Part B is now screened in asimilar manner as Part A.

Combination of Parts A and B

Parts A and B may then be combined and subjected to forming as alreadydescribed above.

The Examples and disclosure are intended to be illustrative and notexhaustive. These examples and description will suggest many variationsand alternatives to one of ordinary skill in this art. All thesealternatives and variations are intended to be included within the scopeof the attached claims. Those familiar with the art may recognize otherequivalents to the specific embodiments described herein which are alsointended to be within the scope of the invention. Therefore, the scopeof the present invention should be interpreted solely from the followingclaims, as such claims are read in light of the disclosure.

What is claimed is:
 1. A cured elastomer composition obtained bycross-linking an uncured blend composition that comprises an intimatelyadmixed mixture that includes the following components:(1) approximately23 to 45 percent by weight of silica that had been silyliated bytreatment and contains trialkylsilyl groups; (2) approximately 55 to 77per cent by weight of a polysiloxane copolymer composed of divalent --R₁R₂ SiO--, divalent --R₃ R₄ SiO-- and end-blocking R₅ R₆ R₇ SiO--units,where R₁ and R₂ independently are lower alkyl of 1 to 6 carbons,phenyl or trifluoropropyl, R₃ is vinyl, allyl, or other olefinic grouphaving up to 4 carbons, R₄ is lower alkyl of 1 to 6 carbons, phenyl ortrifluoropropyl, and R₅, R₆, and R₇ independently are lower alkyl of 1to 6 carbons, phenyl, vinyl, allyl, or other olefinic group having up to4 carbons and one double bond, the polysiloxane copolymer has a degreeof polymerization (D. P.) approximately in the range of 3500 to 6500,and the olefin containing --R₃ R₄ SiO-- groups are present randomlydistributed in the polysiloxane copolymer and approximately in the 0.05to 0.3 mol percent range, with the provisos that when R₁, or R₂, or bothrepresent phenyl groups then proportion of the phenyl-containingdivalent siloxane units does not exceed 15 mol per cent and when R₁ orR₂ or both represent trifluoropropyl groups, then the proportion of thetrifluoropropyl-containing divalent siloxane units does not exceedapproximately 40 mol per cent in the polysiloxane copolymer; (3) acatalyst, and (4) an organohydrogen polysiloxane cross-linker, thecatalyst and the cross-linker being present in the uncured blendcomposition in sufficient amount to cause the cross-linking reaction tooccur.
 2. The cured elastomer composition of claim 1 wherein the --R₁ R₂SiO-- group is --(CH₃)₂ SiO--.
 3. The cured elastomer composition ofclaim 1 wherein the --R₃ R₄ SiO-- group is --CH₃ (CH₂ ═CH)SiO--.
 4. Thecured elastomer composition of claim 1 wherein the R₅ R₆ R₇ SiO-- groupis --(CH₃)₂ (CH₂ ═CH)SiO--.
 5. The cured elastomer composition of claim1 wherein the --R₁ R₂ SiO-- group is --(CH₃)₂ SiO--, the --R₃ R₄ SiO--group is --CH₃ (CH₂ ═CH)SiO--, and the R₅ R₆ R₇ SiO-- group is --(CH₃)₂(CH₂ ═CH)SiO--.
 6. The cured elastomer composition of claim 5 where the--CH₃ (CH₂ ═CH)SiO-- group is present in the polysiloxane copolymer inthe proportion of 0.142 mol percent.
 7. The cured elastomer compositionof claim 1 where the silyliated silica includes trimethylsilyl groups insuch quantity that the carbon content of the silica is in the range ofapproximately 4 to 8 per cent by weight.
 8. The cured elastomercomposition of claim 7 where the carbon content of the silyliated silicais approximately 7.3 per cent by weight.
 9. The cured elastomercomposition of claim 8 where the silyliated silica is present in thecured elastomer in the proportion of approximately 39 per cent byweight.
 10. The cured elastomer composition of claim 1 where thecatalyst is a platinum catalyst.
 11. The cured elastomer composition ofclaim 1 where the organohydrogen polysiloxane cross-linker has theformula (R)_(a) (H)_(b) SiO_(2-a/2-b/2) where R is simple lower alkyland "a" ranges from 1.00 to 3, and "b" ranges from 0.1 to 1.0.
 12. Anuncured elastomer blend composition suitable for curing by crosslinking, the composition comprising two aliquots in combination:thefirst aliquot comprising an intimately admixed mixture of the followingcomponents:(1) approximately 23 to 45 percent by weight of silica thathad been silyliated by treatment and contains trialkylsilyl groups; (2)approximately 55 to 77 per cent by weight of a polysiloxane copolymercomposed of divalent --R₁ R₂ SiO--, divalent --R₃ R₄ SiO-- andend-blocking R₅ R₆ R₇ SiO-- units, where R₁ and R₂ independently arelower alkyl of 1 to 6 carbons, phenyl or trifluoropropyl, R₃ is vinyl,allyl, or other olefinic group having up to 4 carbons, R₄ is lower alkylof 1 to 6 carbons, phenyl or trifluoropropyl, and R₅, R₆, and R₇independently are lower alkyl of 1 to 6 carbons, phenyl, vinyl, allyl,or other olefinic group having up to 4 carbons and one double bond, thepolysiloxane copolymer has a degree of polymerization (D. P.)approximately in the range of 3500 to 6500, and the olefin containing--R₃ R₄ SiO-- groups are present randomly distributed in thepolysiloxane copolymer and approximately in the 0.05 to 0.3 mol percentrange, with the provisos that when R₁, or R₂, or both represent phenylgroups then proportion of the phenyl-containing divalent siloxane unitsdoes not exceed 15 mol per cent. and when R₁ or R₂ or both representtrifluoropropyl groups, then the proportion of thetrifluoropropyl-containing divalent siloxane units does not exceedapproximately 40 mol per cent in the polysiloxane copolymer; and a(3) acatalyst, the second aliquot comprising an intimately admixed mixture ofthe following components:(1) approximately 23 to 45 percent by weight ofsilica that had been silyliated by treatment and contains trialkylsilylgroups; (2) approximately 55 to 77 per cent by weight of a polysiloxanecopolymer composed of divalent --R₁ R₂ SiO--, divalent --R₃ R₄ SiO-- andend-blocking R₅ R₆ R₇ SiO-- units, where R₁ and R₂ independently arelower alkyl of 1 to 6 carbons, phenyl or trifluoropropyl, R₃ is vinyl,allyl, or other olefinic group having up to 4 carbons, R₄ is lower alkylof 1 to 6 carbons, phenyl or trifluoropropyl, and R₅, R₆, and R₇independently are lower alkyl of 1 to 6 carbons, phenyl, vinyl, allyl,or other olefinic group having up to 4 carbons and one double bond, thepolysiloxane copolymer has a degree of polymerization (D. P.)approximately in the range of 3500 to 6500, and the olefin containing--R₃ R₄ SiO-- groups are present randomly distributed in thepolysiloxane copolymer and approximately in the 0.05 to 0.3 mol percentrange, with the provisos that when R₁, or R₂, or both represent phenylgroups then proportion of the phenyl-containing divalent siloxane unitsdoes not exceed 15 mol per cent, and when R₁ or R₂ or both representtrifluoropropyl groups, then the proportion of thetrifluoropropyl-containing divalent siloxane units does not exceedapproximately 40 mol per cent in the polysiloxane copolymer, and(3) anorganohydrogen polysiloxane cross-linker, the catalyst and thecross-linker being present in the first and second aliquots respectivelyin sufficient amounts to cause the cross-linking reaction to occur afterthe first and second aliquots are intimately mixed.
 13. The uncuredelastomer blend composition of claim 12 wherein in the polysiloxanecopolymer of each aliquot the --R₁ R₂ SiO-- group is --(CH₃)₂ SiO--, the--R₃ R₄ SiO-- group is --CH₃ (CH₂ ═CH)SiO--, and the R₅ R₆ R₇ SiO--group is --(CH₃)₂ (CH₂ ═CH)SiO--.
 14. The uncured elastomer blendcomposition of claim 13 wherein in the polysiloxane copolymer of eachaliquot the silyliated silica includes trimethylsilyl groups in suchquantity that the carbon content of the silica is in the range ofapproximately 4 to 8 per cent by weight.
 15. The uncured elastomer blendcomposition of claim 12 wherein the second aliquot further comprises avolatile inhibitor in such quantity that the cross-linking reactionoccurs rapidly after intimately mixing the first and second aliquots andsubstantially removing the inhibitor by heat.